Terms such as clock rate, operating frequency, processor speed, and core rate are often used to describe the performance of a processor. These terms are widely recognized as the megahertz (MHz) or gigahertz (GHz) rating of a computer. In general, a higher clock rate means that a processor can perform more instructions per second.
A variety of factors can contribute to selecting a clock rate for a given processor. For instance, slight imperfections and variations during the manufacturing process can have an impact on how quickly and reliably a processor can be operated. In which case, different processors from the same manufacturing process may support different maximum clock rates. Put another way, several processors may appear interchangeable, but they may each be configured to operate at a different clock rate.
Wafer yield rates tend to be lower for faster processors. That is, the percentage of processors that can support a given clock rate setting tends to decrease as the clock rate setting increases. In which case, the manufacturing costs, and sale prices, tend to be higher for faster processors. These higher prices create an incentive for over-clocking.
Over-clocking refers to operating a processor at a higher clock rate than the processor is configured for. For instance, unscrupulous individuals or companies may purchase less expensive, slower processors and repackage them to run at higher clock rates. Often times, these over-clocked processors are re-sold as more expensive, faster processors.
Over-clocking can be problematic for both the processor manufacturer and the end user. Not only does the manufacturer loose revenue by selling the less expensive processor, but the over-clocked processor may not reliably support the higher clock rate, which can erode the manufacturer's reputation for quality. And, not only does the end user overpay for what he or she actually receives, but the end user may be frustrated by less-than-expected reliability.